This invention relates to a polarization multiplexing optical transceiver to be used for an optical transmission system.
In ultrahigh-speed optical fiber transmission, in order to effectively use a wavelength range (or frequency range) usable for signal transmission, there is widely used wavelength multiplexing transmission configured to: transmit a plurality of optical signals different in wavelength in a bundle through an optical fiber; split the optical signals into original wavelengths at a receiving end; and to receive the optical signals by each optical receiver. As another method for using the frequency range more efficiently, use of polarization multiplexing transmission is under investigation.
Polarization multiplexing is a multiplexing method that utilizes a difference in polarization state of light. Two sets of optical signals modulated by independent information signals on a transmission side are converted into polarization states orthogonal to each other to be multiplexed, and then transmitted through the optical fiber. The polarization state of the optical signal can be represented as a point on a Poincare sphere. On an optical fiber transmission line, the polarization state of the optical signal is subjected to random conversion on the Poincare sphere, but the orthogonality of the polarization states is maintained. Thus, by performing conversion processing of the polarization state and polarization splitting on a receiving side, the two original multiplexed optical signals can be split, and information twice as much can be transmitted by using the same wavelength width.
In recent years, as a candidate of a large-capacity transmission method of 100 giga bits per second (Gb/s) or more in particular, attention is focused on a digital coherent polarization multiplexing transmission method for polarization-multiplexing a multilevel-modulated optical signal to transmit the signal, and polarization-splitting the optical signal to receive the signal by using a digital coherent reception technology. An example of such a method is “Spectrally Efficient Long-Haul Optical Networking Using 112-Gb/s Polarization-Multiplexed 16-QAM” by P. J. Winzer.
FIG. 12 is a diagram illustrating a configuration of a related-art polarization multiplexing optical transceiver 100. In each figure below, a path of parallel digital electric signals is indicated by a white arrow, a path of high-speed serial electric signals is indicated by a thin line, and a path of optical signals is indicated by a thick line.
The related-art polarization multiplexing optical transceiver 100 includes a pair of a related-art polarization multiplexing optical transceiver 101 and a related-art polarization multiplexing optical receiver 130, and is oppositely coupled to a polarization multiplexing optical transceiver of a similar type located at a place of several tens to several thousands of kilometers away via an optical fiber transmission line, thereby achieving long-distance optical fiber transmission.
In the related-art polarization multiplexing optical transceiver 101, a transmission laser beam 114 output from a transmission laser source 113 is split into two continuous wave (CW) laser beams 116-1 and 116-2 by an optical splitter 115, and respectively input to IQ optical field modulators 117-1 and 117-2. An IQ optical modulator (also referred to as IQ modulator) is an optical modulator that includes two sets of Mach-Zehnder (MZ) modulators arranged in parallel on a substrate of lithium niobate or the like. The IQ optical modulator can independently modulate an in-phase component (I component, real part) and a quadrature-phase component (Q component, imaginary part) of an optical field by applying a high-speed modulated voltage signal to a modulation signal input terminal of each MZ modulator.
In the example illustrated in FIG. 12, two high-speed serial signals output from two high-speed digital/analog (DA) converters 112-1 and 112-2 are input to the IQ optical field modulator 117-1. An optical signal having the former as a real part and the latter as an imaginary part is generated to be output as an X-polarized multilevel modulated optical signal 118. On the other hand, two high-speed serial signals output from two high-speed DA converters 112-3 and 112-4 are input to the IQ optical field modulator 117-2. An optical signal having the former as a real part and the latter as an imaginary part is generated to be output as a Y-polarized multilevel modulated optical signal 119.
The high-speed serial signal output from each DA converter corresponds to a real part or an imaginary part of a multilevel information signal. Accordingly, the X-polarized multilevel modulated optical signal 118 and the Y-polarized multilevel modulated optical signal 119 are independent multilevel modulation light beams modulated on a two-dimensional complex plane. The X-polarized multilevel modulated optical signal 118 and the Y-polarized multilevel modulated optical signal 119 are each input to a polarization multiplexer 122, and are output as a polarization multiplexing transmission optical signal 120 from a transmission light output fiber 121.
On the other hand, a transmission information signal 103 to be transmitted is first input to a related-art transmission side digital signal processing unit 102, and header information such as control information is added to the signal 103 by an internal framer 105. Then, the transmission information signal 103 is shaped to a standardized information frame of Optical Transport Network (OTN) 4 or the like. Almost the entire information frame is input to an error correction code generator 104, and error correction information is added thereto. Such information signals are interleaved to be split into an X-polarization information signal 106 and a Y-polarization information signal 107. X-polarization information and Y-theoretically information can be theoretically split for each frame. However, in order to reduce a delay and a circuit size, a method of splitting the information signals at regular intervals as in the case of odd/even bit or byte interleaving is employed.
The X-polarization and Y-polarization information signals 106 and 107 are respectively input to multilevel encoders 108-1 and 108-2, and assigned multilevel symbols for each plurality of bits to be converted into multilevel signal strings. Then, the X-polarization and Y-polarization information signals 106 and 107 are subjected to arithmetic processing (not shown) such as sampling rate conversion or interpolation. When necessary, the X-polarization and Y-polarization information signals 106 and 107 are input to digital pre-equalizing circuits 109-1 and 109-2 for compensating for an influence of transmission impairment such as chromatic dispersion of the optical fiber on the transmission side, and then respectively converted into X-polarization and Y-polarization digital output signals 110 and 111. Although not illustrated, when necessary, each digital output signal is subjected to processing such as linear/nonlinear response compensation at the optical modulator or a high frequency circuit. Then, a real part and an imaginary part of the X-polarization digital output signal 110 are respectively input to the DA converters 112-1 and 112-2, and a real part and an imaginary part of the Y-polarization digital output signal 111 are respectively input to the DA converters 112-3 and 112-4.
FIGS. 13A to 13D are diagrams illustrating a display method of the optical multilevel signal, a signal constellation of optical multilevel modulation, and polarization multiplexing transmission.
FIG. 13A shows a signal point on a complex plane (IQ plane). A modulation state of each multilevel signal can be represented by an optical field (strictly, equivalent low-frequency representation) at center timing (decision timing) of a modulation waveform. The signal point can be represented on complex Cartesian coordinates (IQ coordinates) or polar coordinates using an amplitude r(n) and a phase Φ(n).
FIG. 13B shows a signal constellation of quarternary phase shift keying (QPSK) for transmitting two-bit information (00, 01, 11, 10) in one symbol by using four values (π/4, 3π/4, −3π/4, −π/4) as phase angles Φ(n). A QPSK signal can be generated by, for example, inputting binary electric signals to two input terminals of an IQ optical field modulator 102-1, that is, an input terminal 103-1 for an in-phase component modulation signal and an input terminal 104-1 for a quadrature-phase component modulation signal, and modulating in-phase and quadrature-phase components (I) and (Q) to positive and negative two values. In polarization multiplexing 100 Gb transmission, quarternary phase shift keying is employed for each polarized optical signal. The rate of a binary electric signal is about 28 Gb/s. In the case of a quarternary phase shift keying signal generated by using this signal, two-bit information can be transmitted in one symbol, and thus the transmission rate is 56 Gb/s.
FIG. 13C shows a signal constellation of sixteen-level quadrature amplitude modulation (16QAM) in which information transmission efficiency is higher. In 16QAM, signal points are arranged in a grid-like pattern, and four-bit information can be transmitted in one symbol. In the shown example, upper two-bit values (10xx, 11xx, 01xx, 00xx) are represented on coordinates of a Q axis, and lower two-bit values (xx10, xx11, xx01, xx00) are represented on coordinates of an I axis. Such multilevel signals can be generated by respectively inputting multilevel electric signals (four-level in this example) to the two input terminals I and Q of the IQ optical field modulator 117 illustrated in FIG. 12, and inputting voltage components corresponding to field coordinates (i(t), q(t)) of in-phase and quadrature-phase components.
FIG. 13D shows a concept of polarization multiplexing. A light wave is a kind of electromagnetic wave, and there are two independent orthogonal polarization states (e.g., horizontal polarization and vertical polarization) depending on field vibration directions with respect to a traveling direction. Thus, two optical field components (X-polarization and Y-polarization components in FIG. 13D) can be modulated by separate information signals, and multiplexed to be transmitted. In the above-mentioned 100 Gb transmission, quarternary phase shift keying signals of 56 Gb/s independent of each other are multiplexed on the X-polarization and Y-polarization components to be transmitted, and accordingly a total transmission speed is 112 Gb/s. The transmission speed exceeds 100 Gb/s because of an increase of data caused by encoding and addition of error correction information, and different values may be set depending on methods.
In FIG. 12, as described above, the polarization multiplexed transmission optical signal 120 is transmitted for a long distance through the optical fiber transmission line of several tens to several thousands of kilometers, and subjected to transmission impairment or arbitrary polarization rotation in the midway due to chromatic dispersion or a nonlinear effect of the optical fiber. The polarization multiplexing optical receiver 130 of the related-art polarization multiplexing optical transceiver on the opposite side receives the transmitted optical signal 120. This polarization multiplexing optical receiver 130 uses coherent detection, and utilizes, as a detection reference of a field component of a received polarization multiplexed signal 133 input from a received light input fiber 132, a local laser beam 135 emitted from a local laser source 134 disposed in the receiver.
The received polarization multiplexed signal 133 is input together with the local laser beam 135 to a polarization-diversity optical 90-degree hybrid front end 136. The front end 136 subjects both optical signals to polarization/phase diversity detection, and outputs four resultant serial electric signals (Is: in-phase component of S-polarization component, Qs: quadrature-phase component of S-polarization component, Ip: in-phase component of P-polarization component, and Qp: quadrature-phase component of P-polarization component). S and P are polarization main axes of the receiver. Those high-speed serial signals are respectively sampled by analog/digital (AD) converters 137-1 to 137-4, converted into an S-polarization received digital signal 138 and a P-polarization received digital signal 139, and input to a reception side digital signal processing unit 131.
In the reception side digital signal processing unit 131, the S-polarization and P-polarization received digital signals 138 and 139 are respectively input to transmission impairment compensators 140-1 and 140-2. The transmission impairment compensators 140-1 and 140-2 mainly compensate for the influence of the chromatic dispersion of the transmission line, and output equalized S-polarization and P-polarization digital signals 141 and 142. In FIG. 12, the two transmission impairment compensators 140-1 and 140-2 are illustrated as independent circuits, which is a configuration that takes no consideration for interaction of both polarization components. When nonlinear interaction such as inter-polarization mutual phase modulation is compensated for, both may be configured to be integral.
Then, the equalized S-polarization and P-polarization digital signals 141 and 142 are input to a polarization demultiplexing unit 143. As described above, the polarization of the received polarization multiplexed signal 133 is rotated in the midway of the transmission line, and hence the polarization main axes S and P on the reception side do not match those on the transmission side. The polarization demultiplexing unit 143 calculates polarization rotation, and based on a result of the calculation, restores/splits X-polarization and Y-polarization digital signals 145 and 144 that are polarization components on the transmission side.
For the polarization demultiplexing unit 143, an adaptive butterfly finite impulse response (FIR) filter, which is a ladder filter with a time domain or a frequency domain of several to several tens of taps, can be used. For example, the polarization demultiplexing unit 143 includes four sets of complex FIR filters, and is configured to couple two polarization components of an input and an output in a butterfly pattern. The polarization demultiplexing unit 143 adaptively controls a tap coefficient based on an algorithm such as a constant modulus algorithm (CMA), thereby adaptively splitting the polarization components and equalizing waveforms.
The X-polarization and Y-polarization digital signals 145 and 144 output from the polarization demultiplexing unit 143 are respectively input to frequency offset estimation circuits 146-2 and 146-1. The frequency offset estimation circuits 146-2 and 146-1 and phase estimation circuits 147-2 and 147-1 correct a frequency difference (frequency offset) between the local laser source and a transmission laser source, and input signals having corrected signal phase shifting (rotational shifting on complex plane) to multilevel decision circuits 148-2 and 148-1.
The multilevel decision circuit 148-1 restores an original Y-polarization information signal 149, and the multilevel decision circuit 148-2 restores an original X-polarization information signal 150. Those information signals are interleave-multiplexed again, and then input to an error correction circuit 152. The error correction circuit 152 and a frame removing circuit 151 detect a head of a data frame, correct an error, remove an error code and a header, and restore a received information signal 153 to be output.
In such polarization multiplexing transmission, the two types of polarization X and Y can be used as independent transmission/reception media. Thus, when the amount of information to be transmitted is small, power consumption may be reduced by stopping the polarization multiplexing transmission to transmit information in a single polarization state. As a technology for achieving power saving, a technology disclosed in JP 2011-250291 A is known. An optical transmitter capable of switching between a polarization multiplexing state and a single polarization state, which is disclosed in JP 2011-250291 A illustrated in FIG. 3, reduces power consumption by cutting off power of a polarization multiplexing encoder and a modulator during single polarization. An optical receiver 30 of JP 2011-250291 A illustrated in FIG. 10 stops some circuits.
In JP 2013-055654 A, there are disclosed a method of reducing power consumption by switching a multilevel polarization multiplexed signal to a single polarization multilevel signal capable of transmitting twice as many bits and a method of shutting down at least one of a driver or a signal processor in a transmitter for dual polarization modulation. In JP 2013-055654 A, there is also disclosed a method of reducing power consumption by controlling a receiver to shut down a component for receiving and processing a signal relating to a polarization component in which no information is modulated, for example, by controlling the receiver to stop an operation of digital signal processing (DSP) for processing a main polarized signal carrying no information.